Opposing sides of the debate on the carbon tax aired their views at two separate rallies in central Sydney on Saturday.

An estimated 4,000 people gathered in Hyde Park in the city's CBD to protest against the policy. The crowd was addressed by opposition MPs, including Eric Abetz, as well as radio personalities.

In Belmore Park about 4,000 people rallied, although organisers said upwards of 8,000 people were in attendance to show their support for the policy.

Head of political advocacy group GetUp!, Simon Sheik, says even though the policy has already been flagged they will keep pushing until a carbon price is implemented.

"We know in the past our governments have failed to deliver climate action, despite the best of their promises," he said.

"That's why it's so important that we continue to mobilise as part of this campaign."

Organiser of the rally protesting against the policy, Luke Jenkins, says it is frustrating that their cause has been hijacked along the way by other political interests.

Mr Jenkins says they have been unfairly labelled extremists.

"You know we all have our own political views. Sometimes we have to stand next to somebody who we don't agree with and never would hang with on an ordinary everyday basis," he said.

"But they are people themselves that have families and this will affect them, it'll affect their household budgets."

He says the rally was in part about proving that plenty of ordinary Australians are opposed to a price on carbon.

Carbon cost emerges

The protests come as the Government plays down reports from Treasury which show the tax would cost an average household almost $900 a year.

The documents, released under Freedom of Information laws on Friday, predict a carbon price of $30 per tonne would push up bills by $860 a year, or $16 per week.

Broken down, the figures show electricity prices would rise by $218.40 annually for the average household, gas by $114.40, petrol by $187.20 and food by $88.40.

But Climate Change Minister Greg Combet says concerns about the impact are premature as the amount of $30 per tonne is not set in stone, and the compensation package to households has not been worked out.

"We haven't yet settled all of the design elements, including the starting price for carbon," he said.

"So what these figures are, are various scenarios based upon the design of a previous emissions trading scheme.

"On the household assistance side ... the Government's been very clear that we would have a generous and fair assistance package for low-income households and middle-income households.

"When we have completed all of the design work, including the starting price, the potential price impacts and the assistance package, we will make that available in full detail so that people can properly consider it."

Opposition finance spokesman Andrew Robb says people are concerned and confused about how much they will be affected by the tax, and has accused the Government of introducing "policy by deception".

"What we've seen is, in my view, another huge deception by this Government," he said.

"The Prime Minister's saying there's no point in speculating about the impact of a tax on families, yet all the while the Prime Minister was sitting on Treasury analysis which showed this nearly $1,000 a year increase in prices."

But Mr Combet rejected any suggestion the Government had been deceptive in refusing to release the figures earlier.

"That's all nonsense. We've heard from Mr Robb it's perfectly appropriate when governments are developing policy positions to look at different scenarios," he said.

"These are indicative numbers based upon the design of the carbon pollution reduction scheme in the last term of Parliament, so they are not the modelling that would reflect the Government's current approach."

Despite his position, Mr Combet said it was "not appropriate to speculate" about what the price per tonne of carbon would be.

"These are figures drawn from the previous design of the carbon pollution reduction scheme. They're not to be relied upon nor to be indicative of where the Government might ultimately determine to have a carbon starting price," he said.

"And so it is not appropriate to be trying to scare people as Mr Robb is doing about these particular numbers."